Gas chromatography with means to flow ionization particles into the ionization chamber



United States Patent OllfiCC 3,134,898 Patented May 26, 1964 GAS CHROMATOGRAPHY WITH MEANS T FLOW IONIZATION PARTICLES INTO Tim IONIZA- TION CHAMBER Maurice R. Burnell, Yorba Linda, Richard A. Foster, La Habra, and William S. Gallaway, Fullerton, Calif., assignors to Beckman Instruments, Inc., a corporation of California Filed June 27, 1960, Ser. No. 38,907 16 Claims. (Cl. 250-435) This invention relates to detectors for gas chromatographs and the like and, in particular, to detectors of the ionization type. In general, the operation of the ionization-type of detector depends upon the selective ionization of components present in the eflluent of a chromatographic column or other sample gas stream. One type of detector using this general approach is the hydrogen flame detector, such as that described by McWilliams and Dewar in Gas Chromatography, 1958 edition, Desty, London. Another detector utilizes radioactive isotopes to produce metastable atoms in the sample gas, this instrument being described by Lovelock in Journal of Chromatography, vol. I, p. 35, 1958.

These prior art instruments have a number of disadvantages including the use of dangerous materials and the requirement of complex safety precautions. Also, radioactive sources cause background noise as a result of statistical variations in radiation output. Accordingly, it is an object of the present invention to provide a new and improved ionization type-detector that is simple and inexpensive to manufacture and which is accurate, sensitive and easy to operate without endangering personnel or other equipment and which does not require radioactive materials.

It is a particular object of the invention to provide an ionization-type detector having a housing with an electrongenerating zone and an ionization chamber zone and including ionization-measuring means with an electrode positioned within the ionization chamber zone, means for generating electrons within the electron-generating zone, means for directing a stream of rare gas into the first zone, and means for directing the stream of sample gas into the housing, preferably between the rare gas inlet and the electrode for flow of the sample gas and the rare gas through the ionization chamber zone. The various components of the sample gas stream are ionized in the ionization chamber and the degree of ionization is indicated or recorded by conventional ionization-measuring equipment. The operation of the detector will be described in detail subsequently.

It is a further object of the invention to provide an ionization-type detector utilizing photoelectrons to produce the ionization of the components of the sample gas. Another object is to provide such an instrument wherein the photoelectrons are generated in the electron-generating zone by light impinging on a metallic surface within the zone. A particular object of the invention is to provide such an instrument wherein the light source comprises an ultraviolet lamp positioned within the zone providing the radiation for generating the photoelectrons.

It is an object of the invention to provide an ionizationtype detector having an electron source and an electronaccelerating grid positioned adjacent the electron source to provide high energy electrons for use in the detector. A further object is to provide such a detector wherein the potential source for the ionization-measuring system may be used to supply the potential for the accelerating grid. A specific object is to provide an ionization-type detector including a housing, an ultraviolet light source positioned within the housing for generating photoelectrons from a metallic surface in the housing, an electron-accelerating grid positioned within the housing, ionization-measuring means connected to the grid and the metallic surface, and means for flowing the sample gas through the housmg.

The invention also comprises novel details of construction and novel combinations and arrangements of parts, which will more fully apepar in the course of the following description. The drawings merely show and the description merely describes preferred embodiments of the present invention which are given by way of illustration or example.

In the drawings:

FIG. 1 is a block diagram illustrating the use of the detector with a gas chromatograph;

FIG. 2 shows a preferred form of the detector of the invention;

FIGS. 3-10 show alternative forms of the detector of FIG. 2; and

FIGS. 11 and 12 show additional alternative forms utilizing an accelerating grid adjacent the electron source.

The detector of the invention is described herein in conjunction with a gas chromatograph but it is not limited to such application and can be used in the analysis of any sample gas stream having ionizable components. The detector can be used to monitor impurities during preparation of noble gases and can be used in air pollution measurements, explosive vapor detection and the like. FIG. 1 shows a typical arrangement of a gas chromatograph including a pressure regulator or flow controller 15, a sample injector 16, the chromatographic column 17, a detector 18, and a recorder 19. The flow of the carrier gas, ordinarily argon although other gases such as helium may be used, is controlled by the pressure regulator 15. At a particular time, a quantity of sample is injected into the carrier gas at the sample injector 16 and the components of the sample are separated as the sample moves through the column. The detector provides an output indicating the time required for each component to pass through the column and also a quantitative measure of the component. The detector output is ordinarily recorded in some form for subsequent review, although the output may merely be indicated for contemporaneous visual mspection.

The detector is shown in detail in FIG. 2 and includes a housing 22 having an electron-generating zone 23 and an ionization chamber zone 24. A source of electrons is provided within the electron-generating zone. In the preferred form shown herein, an ultraviolet lamp 25 is,

mounted in a socket 26 in a plug 27, the lamp being energized from a power supply 28. The housing includes a metallic surface that produces photoelectrons when radiation from the lamp impinges thereon. Typically, the housing may be a copper tube which provides the mechanical support for the detector as well as acting as the metallic surface. Conventional ionization-measuring means are provided, including a power supply 31 and an electrometer type amplifier 32, with the wall of the metal housing and an electrode 33 serving as the ionization chamber, the electrode being mounted in an insulating plug 34. A gas outlet line 35 is provided for exhaust from the ionization chamber zone. A gas inlet line 36 is pro vided for directing a stream of gas into the electron-generating zonel Another gas inlet line 37 is provided for directing the sample gas from the chromatographic column into the ionization chamber zone.

In one specific embodiment of the detector of FIG. 2, the ionization chamber zone 24 is formed of inch O.D. copper tubing to provide a chamber having a volume. of about one milliliter. The lamp 25 was a Pen- Ray No. 11-SC-1 and the ionization chamber zone 24 was a length of A inch O.D. copper tubing. A 2,000 volt variable power supply was used in conjunction with a Kiethley Model No. 410 micro-microammeter for the 3 amplifier. The detector of the invention is used in chrmatographic analyses in the same manner as conventional detectors.

While the operating mechanism of the detector is not completely established, it presently appears that the following description is correct. Bombardment of the metallic surface of the housing by energetic photons causes emission of electrons. of low energy, very little more than thermal energy. In an atmosphere of the rare gases, these electrons can acquire energies several hundred times thermal energy when subjected to an electrical field. For example, it has been shown that electrons in argon may acquire root mean square energies of 330 times thermal energy. With thermal energy of 0.038 elec tron volt at room temperature, the electrons acquire a R.M.S. energy of slightly over 12 electron volts. The energy distribution for such electrons in argon has been determined and some electrons exist with energies over twice the R.M.S. value. The electrons generated in the electrongenerating zone 23 diffuse into the ionization chamber with the argon stream and mix with the sample gas of the column. When in the electric field of the ionization chamber, these electrons acquire considerably higher energy and produce ionization of the components of the sample stream. However, electrons lose this acquired energy by elastic and inelastic collisions with argon and other gases present. Since the first excitation level of argon is 11.6 electron volts, the argon acts as an energy sink holding the electron energy to somewhat more than this level. Thus little ionization of the carrier gas occurs but gases with lower ionization potentials are ionized and thereby readily detected. Both the theory referred to above and actual analysesperformed indicate that the more highly energized electrons produce ionization of substances which ionize at 12 electron volts and higher, such as methane and ethane. The operation of the ionization chamber is somewhat similar to that of a proportional Geiger counter in that the electrode is positively charged so that electrons resulting from ionization of components of the gas sample are collected at the electrode. By increasing the potential of the ionization chamber, gas multiplication of electrons is obtained thereby yielding a higher signal.

The two-zone or two-chamber housing of the detector of FIG. 2 provides a. relatively large zone 23 for the electron source, here the ultraviolet lamp 25 and the metal wall of the housing, and a relatively small volume zone for the ionization chamber. The minimum size of the ionization chamber is determined by the breakdown potential between the centrally positioned electrode and the wall of the chamber. The sensitivity of the detector is approximately proportional to the volume of the ionization chamber. However, the volume must not be so large as to produce broadening of the peaks due to dispersion of the sample component in the carrier. The purge gas helps to sharpen the peaks or give a lower effective volume by sweeping the sample components through at a faster rate. It is also desirable to provide the electron source separate from the ionization chamber as some components in the sample gas stream tend to absorb radiation which, in turn, could change the flux of emitted electrons and, hence, the background current and sensitivity of the detector. Also, some of the heavy components in the sample gas as well as eluted column substrate tends to coat the interior of the conduits, which coating would affect the emission characteristics of the electron-emitting surface. The separate inlet in the electron-generating zone provides a purge stream which carries the electrons into the ionization chamber and prevents sample components from entering the electrongenerating zone.

The surface on which the light falls in the generating zone must be photoemissive to at least a portion of the available spectrum. For example, the Pen-Ray ultraviolet lamp No. llSC-1 emits a strong line at 2537 A.

Since this 2537 line is equivalent to about 4.9 electron volts energy, a metallic surface is necessary which has a work function below this value. The alkalis and alkaline earths are unsuitable because of their reactivity but other practical possibilities include aluminum, bismuth, cadmium, cobalt, copper, lead, molybdenum, silver, tantalum, tin and zinc. Some of the metals form oxides having a work function similar to that of the metal and the metallic surface may be in the oxide form. Copper is preferred as it is readily available in many shapes and oxidation does not appreciably alter the emission of photoelectrons. If a metal with a low work function, such as potassium or cesium, was used, light in the visible range would sufiice. Also, X-rays can be used with any metal to produce the photoelectrons.

In the preferred embodiment described above, argon is used as the purge gas although the other rare gases, helium, neon, krypton and xenon would be useable provided they are sufliciently pure. The purge gas as well as the carrier gas in the chromatograph should have an ionizing potential sufiiciently high so as not to be ionized by the highest energy electron produced in the ionization chamber. Argon is especially desirable since common impurities present are not ionized at the first excitation level of argon. Consequently, electrons do not acquire sufficient energy to ionize the impurities to an undesirable extent. Commercial helium contains some ionizable impurities which adversely affect the detector operation. The first excitation potential of helium is so high that all other gases except possibly neon are ionized. The resulting background current from impurities thus obscures the sample components. The other gases are too expensive for ordinary use.

The particular arrangement of the physical components of the detector is not critical. The embodiment of FIG. 3 is similar to that of FIG. 2 and includes an inwardly directed ridge 40 at the junction between the two zones of the housing and a bafiie plate 41 carried on the inlet line 37 adjacent the end thereof. This particular structure produces more intimate mixing of the two incoming gas streams and also substantially eliminates any diffusion of the sample gas from the column into the electron-generating zone 23. The embodiments of FIGS. 2 and 3 are generally referred to as in-line units. FIG. 4 shows a similar structure in which the gas flow path through the electron-generating zone 23 is at a right angle to the gas flow path through the ionization chamber zone 24. This structure is ordinarily referred to as an L unit.

Two forms of T units are shown in FIGS. 5, 6 and 7. In the detector of FIG. 5, the axes of the tubular structures forming the two zones of the housing lie in the same plane, while in the structure of FIGS. 6 and 7, the axes are displaced from each other. In the embodiment of FIGS. 8 and 9, the electron-generating zone 23is concentrically positioned about the ionization chamber zone 24 and a circular lamp 42 is substituted for the straight lamp 25. In one specific form of this embodiment, a Pen-Ray No. 11SC5 lamp was utilized.

FIG. 10 shows another form of the detector manufactured from a solid block of copper 70. A chamber 71 is formed inside the block 70 by suitable means such as drilling. A tubular member 72, which serves as the wall of the ionization chamber, projects from a base 73 which, in turn, is clamped against one face of the block 70 by a plate 74. An insulating washer 75 is positioned between the plate 74 and base 73, the central electrode 33 being supported in the insulating washer within the tubular member 72. The line 37 from the chromatographic column passes through a pipe fitting 76 in the block 70 and terminates at the open end of the tubular member 72. The line 36 supplying the purge stream of argon terminates in another fitting 77 and communicates with the chamber 71. The lamp 25 is supported in a sleeve 78 carried in a pipe fitting 79 in the block, the lamp being positioned in the chamber 71 above the line 37. It is preferred to have the tubular member 72 flared somewhat, providing better gas flow characteristics within the detector. The exhaust line 35 is carried on the base 73 and communicates with the interior of the tubular member 72. This design provides a compact and rugged detector which is easily manufactured, assembled and disassembled and one which is well shielded from external interference. The detectors of FIGS. 3-10 are connected and operated in the same manner as the detector of FIG. 2.

Another alternative form of the detector which is desirable in certain applications is shown in FIG. 11. The detector is shown as a T unit having a first tube 43 serving as the electron-generating zone and a second tube 44 serving as the ionization chamber zone. The effluent from the chromatograph column is directed through the ionization chamber from left to right as shown in the figure. Conventional ionization-measuring means including a power supply 45, an electrometer amplifier 46, and an indicator or recorder 47 are used, with one input of the amplifier connected to the tubular body 44 and the positive terminal of the power supply connected to an electrode 48 which is positioned within the ionization chamber zone by an insulator 49.

An electron source, which may be identical to the electron source of the embodiment of FIG. 2, is contained in the electron-generating zone 43. Means are also included in the electron-generating zone for estabhshing an electric field within the zone for substantially increasing the energy of the electrons generated therein. In the embodiment of FIG. 11, a metal screen or grid 50 is positioned around the lamp 25 within the tubular portion 43, the grid being mounted on the insulating plug 27. An electron-accelerating potential source 51 is connected between the metal Wall and the screen. The high energy electrons produced in the zone 43 are swept through the opening 52 into the zone 44 by the gas stream entering through the line 36. The screen or grid 50 is positive relative to the wall of the chamber and, hence, may be termed an anode. The anode when used is ordinarily operated at a potential of about 800 volts relative to the wall of the tube.

It appears that some metastable atoms of the purge gas, here argon, are produced in the electron-generating zone by the higher energy electrons. The metastable argon atoms will also be swept into the ionization chamber and will produce ionization of components of the sample by collision in the same manner as the high energy electrons previously referred to. While metastable argon atoms may be desirable in certain applications of the detector, it appears that the electrons are the important source of ionization of the components of the sample. The translational velocity of the average electron in the electrical field is about 5,000 times that of the argon, since the latter moves with thermal energy only. Thus, an electron would have a better chance of collision than an argon atom. In going to its metastable state, the argon behaves as an energy sink for the highest energy electrons, so that few electrons acquire sufiicient energy to ionize the carrier gas, while the electrons have enough energy to ionize the components of the sample. The separate accelerating anode of the embodiment of FIG. 11 is not necessary to the operation of the instrument, as the positive electrode of the ionization-measuring means can supply the necessary field for increasing the energy of the electrons. The grid 50 may be used to control or reduce the background current by capturing some of the photoemitted electrons.

Other forms of electron sources may be substituted for the ultraviolet lamp 25. For example, a mercury arc, hydrogen lamp, Xenon lamp, or the like, may be used to 6 supply radiation for impinging on a metallic or metal oxide surface to produce photoelectrons, depending upon the work functions of the surface. Alternatively, a heated filament or an X-ray source could be used.

Another form of the detector utilizing the accelerating anode is shown in FIG. 12. A metal tube 60 is closed at each end with insulating plugs 61, 62 respectively. An inlet line 63 from the chromatograph column is positioned in one of the plugs, here the plug 62, and an exhaust line 64 is positioned in the other plug for sample gas flow through the tube. The ultraviolet lamp25 is mounted in the socket 26 carried in the plug 62. An accelerating anode, such as the grid 50 is carried between the plugs 61, 62 and is positioned about the lamp 25. The ionization-measuring means of the embodiment of FIG. 11 may be utilized with the positive side of the supply 45 connected to the anode 50 and the other terminal connected to the tube 60. In this particular embodiment, the electron source and the ionization chamber are in a single volume which provides a mechanically simpler instrument but one which does not have all the advantages of the earlier described embodiments. This embodiment does permit the use of an electron-accelerating anode adjacent the electron source without requiring a separate power supply.

It should be noted that while all of the embodiments herein have been shown as made from metal tubing, this structure is not mandatory. For example, glass tubing or other materials may be used, it merely being necessary that a metallic surface be present in the electron-generating zone from which electrons can be emitted. In one specific detector, a glass tube was used, the interior of the tube being lined with metal foil. Similarly, the ionization chamber must have a positive and negative electrode but could consist of a glass tube with the two electrodes suspended therein. Also, it should be noted that the electron-generating zone could be provided witli a glass or quartz Wall with the metal surface inside the zone and the light source outside the zone, with radiation from the light source directed through the glass Wall to the metal surface.

Although exemplary embodiments of the invention have been disclosed and discussed, it will be understood that other applications of the invention are possible and that the embodiments disclosed may be subjected to various changes, modifications and substitutions without necessarily departing from the spirit of the invention.

We claim as our invention:

1. In an ionization-type detector for a gas chromatograph or the like producing a sample gas stream for analysis, the combination of: a housing including an electrongenerating section and an ionization chamber with a flow path therebetween; ionization-measuring means for generating an electric field within said chamber; a source of low energy electrons within said electron-generating section; a first gas inlet for directing a stream of rare gas into said electron-generating section of said housing; a gas outlet from said chamber; and a second gas inlet for directing the stream of sample gas into said chamber, for flow from said first and second inlets, through .said ionizatron chamber, and out said outlet, with said stream of rare gas carrying electrons into said chamber and mixing with the sample gas stream. a

2. In an ionization-type detector for a gas chromatograph or the like producing a sample gas stream for analysis, the combination of: a closed housing having a first zone and a second Zone; ionization measuring means including an electrode positioned Within said second zone; means for generating electrons within said first zone; a gas outlet from said second zone; first means for directing a stream of rare gas into said first zone for carrying electrons from said first zone to said second zone; and second means for directing the stream of sample gas into said housing for mixing with said stream of rare gas in said second zone.

3. A detector as defined in claim 2 in which the gas flow path through said first zone is in alignment with the gas flow path through said second zone, and with said second gas stream-directing means positioned adjacent the boundary between said zones and directed into said second zone.

4. A detector as defined in claim 2 in which the gas flow path through said first zone is substantially at a right angle to the gas flow path through said second zone with said second zone at one end of said first zone remote from said first gas stream-directing means, and with said second gas stream-directing means positioned adjacent the boundary between said zones and directed into said second zone.

5. A detector as defined in claim 2 in which the gas flow path through said first zone is substantially at a right angle to the gas flow path through said second zone with said second zone coupled to said first zone remote from the ends of said first zone, and with said second gas stream-directing means positioned adjacent the boundary between said zones and directed into said second zone.

6. A detector as defined in claim 2 in which said first zone is concentrically disposed about said second zone, and with said second gas stream-directing means positioned adjacent the boundary between said zones and directed into said second zone.

7. In an ionization-type detector for a gas chromatograph or the like producing a sample gas stream for analysis, the comibnation of: a closed housing having a first zone and a second zone, with said zones meeting at a boundary; ionization measuring means including an electrode positioned within said second zone; means for generating electrons in said first zone of an energy that is very low relative to that of beta rays for selective ionization of a gas mixture; a gas outlet from said second zone; first means for directing a stream of rare gas into said first zone for carrying electrons from said first zone to said second zone; and second means positioned adjacent said boundary for directing the stream of sample gas into said second zone for mixing with said stream of rare gas in said second zone.

8. In an ionization-type detector for a gas chromatograph or the like producing a sample gas stream for analysis, the combination of: a closed housing having a first zone and a second zone; ionization measuring means including an electrode positioned within said second zone; means for generating photoelectrons within said first zone; a gas outlet from said second zone; first means for directing a stream of rare gas into said first zone for carrying photoelectrons from said first zone to said second zone; and second means for directing the stream of sample gas into said housing for mixing with said stream of rare gas in said second zone.

9. In an ionization-type detector for a gas chromatograph or the like producing a sample gas stream for analysis, the combination of: a closed housing having a first relatively large volume zone and a second relatively smallvolume zone, with said zones meeting at a boundary; means for producing an electric field within said second zone; means for generating electrons in said first zone of an energy that is very low relative to that of beta rays for selective ionization of a gas mixture; a gas outlet from said second zone; first means for directing a stream of rare gas into said first zone for carrying electrons from said first zone to said second zone for acceleration by said electric field; and second means positioned adjacent said boundary for directing the stream of sample gas into said second zone for mixing with said stream of rare gas in said second zone.

10. In an ionization-type detector for a gas chromat- \ograph or the like producing a sample gas stream for analysis, the combination of: a closed housing having a first zone and a second zone; means for generating photoelectrons within said first zone of slightly more than thermal energy; a gas outlet from said second zone; first means for directing a stream of rare gas into said first zone for carrying electrons from said first zone to said second zone; means for producing an electric field in said second zone for accelerating said photoelectrons to an energy level in the order of the first excitation potential of said rare gas; and second means for directing the stream of sample gas into said housing for mixing with said stream of rare gas in said second zone.

11. In an ionizaiton-type detector for a gas chromatograph or the like producing a sample gas stream for anaylsis, the combination of: a closed housing having a first zone and a second zone; ionization measuring means including an electrode positioned within said second zone; a metallic surface in said first zone; a light source directed onto said metallic surface for generating photoelectrons in said first zone; a gas outlet from said second zone; first means for directing a stream of rare gas into said first zone for carrying electrons from said first zone to said second zone; and second means for directing the stream of sample gas into said housing for mixing with said stream of rare gas in said second zone.

12. In an ionization-type detector for a gas chromatograph or the like producing a sample gas stream for analysis, the combination of: a closed housing having a first zone and a second zone; ionization measuring means including an electrode positioned within said second zone; an ultraviolet lamp positioned within said first zone, said housing including a metallic surface in said first zone for generating photoelectrons when light from said lamp impinges thereon; a gas outlet from said second zone; first means for directing a stream of rare gas into said first zone for carrying electrons from said first zone to said second zone; and second means for directing the stream of sample gas into said housing for mixing with said stream of rare gas in said second zone.

13. In an ionization-type detector for a gas chromatograph or the like producing a sample gas stream for analysis, the combination of: a closed housing having a first zone and a second zone; ionization measuring means including an electrode positioned within said second zone; a source of electrons of relatively low energy within said first zone; an electron accelerating anode positioned within said first zone; an accelerating potential source connected between said anode and said electron source for accelerating electrons to an energy level in the order of the ionization potential to said rare gas; a gas outlet from said second zone; first means for directing a stream of rare gas into said first zone for carrying electrons from said first to said second zone; and second means for directing the stream of sample gas into said housing for mixing with said stream of rare gas in said second zone.

14. In an ionization-type detector for a gas chromatograph or the like producing a sample gas stream for analysis, the combination of: a closed housing having a first zone and a second zone; ionization measuring means including an electrode positioned within said second zone; a light source directed into said first zone; a metallic surface in said first zone for generating photoelectrons when light from said source impinges thereon; an electron accelerating anode positioned within said first zone; an accelerating potential source connected between said anode and said metallic surface for accelerating electrons to an energy level in the order of the ionization potential of said rare gas; a gas outlet from said second zone; first means for directing a stream of rare gas into said first zone for carrying electrons from said first zone to said second zone; and second means for directing the stream of sample gas into said housing for mixing with said stream of rare gas in said second zone.

15. A method of detecting constituents in a gas sample such as the eluent of a gas chromatograph, including the steps of: flowing the sample gas through an ionization measuring space; generating a supply of low energy electrons remote from the ionization measuring space; moving the electrons into the ionization measuring space and mixing the electrons with the sample gas in the ionization measuring space; accelerating the electrons in the ionization measuring space to an energy level for selective ionization of constituents of the sample gas; and measuring the ionization current in the ionization measuring space.

16. A method of detecting constituents in a gas sample such as the eluent of a gas chromatograph, including the steps of: flowing the sample gas into an ionization measuring space; directing a light source onto a metallic surface remote from the ionization measuring space for generating photoelectrons; sweeping the electrons into the ionization measuring space with a rare gas for mixing the electrons and rare gas with the sample gas in the ionization measuring space, with the rare gas also moving the sample gas through the ionization measuring space; accelerating the electrons in the ionization measuring space to an energy level in the order of the ionization potential of the rare gas; and measuring the ionization current in the ionization measuring space.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Deal et al.: A Radiological Detector for Gas Chromatography, Analytical Chemistry, vol. 28, No. 12, April 1956, pages 1958 to 1964. 

1. IN AN IONIZATION-TYPE DETECTOR FOR A GAS CHROMATOGRAPH OR THE LIKE PRODUCING A SAMPLE GAS STREAM FOR ANALYSIS, THE COMBINATION OF: A HOUSING INCLUDING AN ELECTRONGENERATING SECTION AND AN IONIZATION CHAMBER WITH A FLOW PATH THEREBETWEEN INIZATION-MEASURING MEANS FOR GENERATING AN ELECTRIC FIELD WITHIN SAID CHAMBER; A SOURCE OF LOW ENERGY ELECTRONS WITHIN SAID ELECTRON-GENERATING SECTION; A FIRST GAS INGLE FOR DIRECTING A STREAM OF RARE GAS INTO SAID ELECTRON-GENERATING SECTION OF SAID HOUSING; A GAS OUTLET FROM SAID CHAMBER; AND A SECOND GAS INLET FOR DIRECTING THE STREAM OF SAMPLE GAS INTO SAID CHAMBER, FOR FLOW FROM SAID FIRST AND SECOND INLETS, THROUGH SAID IONIZATION CHAMBER, AND OUT SAID OUTLET, WITH SAID STREAM OF RARE GAS CARRYING ELECTRONS INTO SAID CHAMBER AND MIXING WITH THE SAMPLE GAS STREAM. 